1. Field of the Invention
The invention relates to a cathode in which electrons are emitted by applying a voltage to a position in the vicinity of a minute-projection, an electron gun having the cathode, and a cathode-ray tube having the electron gun.
2. Description of Related Art
FIG. 1 is a schematic section view showing a configuration of a monochrome cathode-ray tube. In the figure, a vacuum enclosure 15 which is made of glass and has a shape of a conventional cathode-ray tube consisting of a panel portion 28, a funnel portion 16, and a neck portion 2. Layers such as a fluorescent layer 18, and a metal back layer 19 are formed on the inner face of a face plate 17 of the panel portion 28 so that these layers constitute a fluorescent screen. An electron gun 1 is disposed in a sealed manner in the neck portion 2. The funnel portion 16 is provided with an anode button 20 through which a high voltage is applied to the electron gun 1. A deflection yoke 22 is attached to the outside of the vacuum enclosure 15 in the vicinity of the portion where the funnel portion 16 and the neck portion 2 are joined with each other. Furthermore, a getter 23 is attached to the inside of the vacuum enclosure 15 through a metal spring 24.
Electron rays 21 are emitted from the electron gun 1 and accelerated and focused by a high voltage of about 20 to 30 KV which is supplied through the anode button 20 from an external high voltage source The electron rays 21 are then electromagnetically deflected by a magnetic field due to the deflection yoke 22 to impinge on the fluorescent screen, whereby the fluorescent layer 18 is excited to emit light.
In the operation of the cathode-ray tube, the degree of vacuum of the vacuum enclosure 15 is important in relation to operation properties and life properties. In a production process of a cathode-ray tube, therefore, the whole of the vacuum enclosure 15 is externally heated (about 400.degree. C.) so as to be sufficiently degassed. After the vacuum enclosure 15 is evacuated in this way, the degree of vacuum of the vacuum enclosure 15 is enhanced to the level of 5.times.10.sup.-5 Torr. Then, the getter 23 is used to further enhance the degree of vacuum to the level of 10.sup.-8 Torr. The getter 23 has a configuration in which a metal such as barium (Ba) is filled in a metal ring. When a high frequency heating is conducted from the outside of the vacuum enclosure 15, the metal such as barium (Ba) is scattered or evaporated to enhance the degree of vacuum. Immediately after the getter material is scattered or evaporated in the vacuum enclosure 15, the degree of vacuum of the vacuum enclosure 15 is slightly lowered. Since residual gas molecules within the vacuum enclosure 15 are thereafter adsorbed by the active getter material, the degree of vacuum of the vacuum enclosure 15 is gradually enhanced. In this case, when a current aging is conducted while emitting electron rays from the electron gun 1, residual gas molecules within the vacuum enclosure 15 are ionized to be activated, and therefore the adsorption on the getter material is promoted so that the degree of vacuum of the vacuum enclosure 15 is abruptly enhanced. This getter effect causes the degree of vacuum of the vacuum enclosure 15 to finally reach the level of 10.sup.-8 Torr. When the degree of vacuum of the vacuum enclosure 15 is at the level of 10.sup.-8 Tort, there arises no problem in operation properties and life properties of the cathode-ray tube.
FIG. 2 is a schematic enlarged section view showing only the neck portion 2 to illustrate in detail the configuration of the electron gun 1. The electron gun 1 may have various electrode configurations. As a typical example, a bi-potential electron gun 1 comprises a cathode which functions as an electron source, electrodes for pulling out electron rays from the cathode and accelerating them, and electrodes for focusing the emitted electron rays on the fluorescent screen. The cathode has a configuration in which a cylindrical base metal 26 is bottomed in the side of the panel portion 28, a cathode material 25 made of barium oxide (Ba.sub.2 O.sub.3), and the like is applied to the outer surface of the bottom portion of the base metal 26, and a heater 27 is disposed inside the bottom portion. The cathode material 25 is heated by the heater 27 to about 750.degree. to 800.degree. C. to emit thermoelectrons. The emitted thermoelectrons are controlled and accelerated by a first grid electrode 3, a second grid electrode 9, and a third grid electrode 10, and then focused by a main electron lens formed between the third grid electrode 10 and a fourth grid electrode I1, to be impinged in a spot-like manner on the fluorescent layer 18 of the fluorescent screen. The electron rays cause the fluorescent layer 18 in FIG. 1 to be excited to emit light, resulting in that an image is produced on the fluorescent screen. The electron gun 1 having such a configuration is fixed at the side of the fourth grid electrode 1i to the inner wall of the neck portion 2 by a contactor 12. The contactor 12 guides to the fourth grid electrode 11 the high voltage which is generated by the external high voltage source and guided through the anode button 20 and a conductive dag 102. The conductive dag is applied on the inner wall of the vacuum enclosure 15. Voltages which are to be respectively applied to the electrodes other than the fourth grid electrode 11, i.e., the first grid electrode 3, the second grid electrode 9, the third grid electrode 10, and the cathode (the base metal 26, and the heater 27) are introduced through lead-in terminals 101 disposed in the bottom of the neck portion 2.
In the electron gun 1 using such a hot cathode, the level of the emission current is restricted by the properties of the cathode material 25. Therefore, it is difficult to sufficiently satisfy recent requirements for a large-sized and high bright cathode-ray tube. In this system, furthermore, the cathode material 25 must be heated to a considerably high temperature, and therefore a counter-measure against heat must be taken in electrodes surrounding the cathode material 25. For example, it is required to prevent the electrodes from being thermally deformed. In some cases, furthermore, the heating of the cathode material 25 to a high temperature causes a part of the cathode material 25 to be evaporated to adhere to the surrounding electrodes, thereby producing a problem in withstand voltage.
In order to solve these problems, a cathode-ray tube using a cold cathode is disclosed in Japanese Patent Application Laid-Open No. 48-90467. FIG. 3 is a schematic section view showing a configuration of the electron gun 1 using a cold cathode. In the cold cathode 5, a silicon (Si) substrate 7 is fixed onto a cold cathode pedestal 4. A number of conical minute projections 7a are formed on the surface of the silicon substrate 7 by a photolithography process (the pitch of the microcones is 1 to 10 .mu.m). Also, column-like minute projections 7b are formed on the surface. An electron pulling electrode 6 is formed at the upper portion of each of the minute projections 7b by a similar photolithography process in such a manner that it is in proximity to the tips of the minute projections 7a. The electron pulling electrodes 6 are connected to a lead wire 8. In the same manner as most of the other electrodes, a voltage is applied to the electron pulling electrodes 6 through the lead-in terminals 101 disposed in the bottom of the neck portion 2. The configuration except the cathode is identical with the cathode-ray tube shown in FIGS. 1 and 2. Therefore, corresponding elements are designated by the same reference numerals and their description is omitted.
In the cold cathode 5, since the electron pulling electrodes 6 and the minute projections 7a consisting of microcones are close in distance to each other, the electric field is caused to concentrate at the tips of the minute projections 7a only by applying a voltage of about several tens to one hundred volts to the electron pulling electrodes 6, so that electrons are emitted from the tips. In the same manner as the case of the conventional hot cathode, the electrons pulled out by the electron pulling electrodes 6 are controlled and accelerated by the first grid electrode 3, the second grid electrode 9, and the third grid electrode 10, and then focused by the main electron lens formed between the third grid electrode 10 and the fourth grid electrode 11. The electron rays are then impinged in a spot-like manner on the fluorescent layer 18 of the fluorescent screen, whereby causing the fluorescent layer 18 to be excited to emit light, resulting in that an image is produced on the fluorescent screen.
FIG. 4 is a schematic section view of a main portion showing the case where such a cold cathode is applied to a flat cathode-ray tube. A vacuum enclosure 29 of the flat cathode-ray tube has a substantially box-like shape, and which comprises a front glass plate 30 and a back glass plate 14. In the case of a color display, a fluorescent screen on which G (green) fluorescent dots 31, B (blue) fluorescent dots 32, and R (red) fluorescent dots 33 are arranged in a mosaic manner is formed on the inner face of the front glass plate 30. On the mosaic-like fluorescent screen, disposed is a metal back film 34 which is made of Al (aluminum) and functions as an anode and a light reflecting film. Cold cathode groups 36 for exciting the G (green) fluorescent dots 31, cold cathode groups 37 for exciting the B (blue) fluorescent dots 32, and cold cathode groups 38 for exciting the R (red) fluorescent dots 33 are formed on the inner face of the back glass plate 14, in such a manner that they respectively correspond to the G, B and R fluorescent dots 31, 32 and 33. In the same manner as the cold cathode shown in FIG. 3, each of the cold cathode groups 36, 37 and 38 comprises a number of minute projections 7a and electron pulling electrodes 6 which are in proximity to the minute projections 7a The electron pulling electrodes 6 are arranged in an X-Y matrix form so that the cold cathode groups 36, 37 and 38 can control the amount of respective electron rays emitted toward the G, B and R fluorescent dots 31, 32 and 33. It is not required to interpose any thing between the front glass plate 30 on which the fluorescent screen is formed, and the back glass plate 14 on which the cold cathode groups 36, 37 and 38 are formed. Therefore, the distance between the front glass plate 30 and the back glass plate 14 can be reduced to a very small value. Consequently, this technique is recently expected to realize an ultra-thin flat cathode-ray tube. The degree of vacuum of the vacuum enclosure 29 which is defined by the front glass plate 30 on which the fluorescent screen is formed, and the back glass plate 14 on which the cold cathode groups 36, 37 and 38 are formed is important in the view points of the electron emission properties of the cold cathodes. When a getter similar to that described above is used, the space within the vacuum enclosure 15 can be maintained at a high
In such cathode-ray tubes, i.e., the cathode-ray tube which has the electron gun 1 using the cold cathode 5 according to the electric field emission, and the flat cathode-ray tube using the cold cathode 5 in which the cold cathode groups are arranged in a mosaic manner corresponding to t, he fluorescent dots, the influence of a small amount of residual gas molecules and acting on the electron emission of the minute projections 7a of the cold cathode 5 cannot be neglected even when the internal space of the vacuum enclosure 15 or 29 is maintained at a considerably high vacuum as described above. One of the causes of this phenomenon is the fact that the cold cathode 5 easily adsorbs gas molecules because it is configured by arranging a number of minute microcones (having a size of, for example, 1 to 3 .mu.m) in the pitch of, for example, several microns and therefore the whole surface area of the cathode is very large.
In the cathode-ray tube which the electron gun 1 using the cold cathode 5 shown in FIG. 3 and according to the electric field emission, even when the degree of vacuum of the vacuum enclosure 15 is maintained at a high vacuum or at the level of 10.sup.-8 Tort by the above-mentioned effect of the getter, and the like, adsorbed gas molecules are gradually released from the fluorescent screen struck by the electron rays 21, the inner surface of the glass plate of the vacuum enclosure 15, and the vicinity of each electrode of the electron gun 1. The released gases are again adsorbed by the tips of the minute projections 7a of the cold cathode 5. Particularly, the electron emission portion of the tip of each minute projections 7a is sharpened to an atomic level, and therefore may largely be affected even when it adsorbs several gas molecules. This causes the amount of electron rays emitted from the electron gun 1 to be unstable, and the amount itself to be reduced. As a result, there arise problems that the light emission of the fluorescent screen due to the electron rays becomes unstable, and that the light emission brightness of the fluorescent screen is lowered.
In the same manner as the case described above, also in the case of the flat cathode-ray tube shown in FIG. 4 and using the cold cathode, adsorbed gas molecules are gradually released from the fluorescent screen, and the inner surface of the vacuum enclosure 29, and again adsorbed by the tips of the minute projections 7a. This causes the amount of electron rays emitted from the cold cathode groups 36, 37 and 38 to be unstable, and the amount itself to be reduced, As a result, there arise problems that the light emission of the fluorescent screen due to the electron rays becomes unstable, and that the light emission brightness of the fluorescent screen is lowered.